August 19 is 4 months after March. That's a lot of time for details to have finally been worked out about how MCAS actually worked.
Also, they did cutoff the electric trim, but had to reenable it in order to try to neutralize already present mistrim.
If the switches had been left alone from the NG, where the flight computer could be isolated while maintaining the electric trim controls, they could have been fine.
...Of course, if Boeing had actually mentioned MCAS in training materials, then things may have been different too.
Knowledge of MCAS was not required for (1) and (2) to work and be effective. Note that the pilots were already doing (1). And the Stab Trim Cutoff switches are there to cut off the stab trim.
Besides, at the time of the Ethiopian crash, the existence of MCAS was well known:
I find this baffling. The stabilizer was clearly running and dangerously pitching the nose down.
The pilots could have safely resolved the situation by:
1. using the electric trim switches to override the runaway trim, which they did
2. then cutting off the stabilizer trim with the cutoff switches, which they did not
(1) and (2) are true. See Aviation Week Aug 19.